This invention relates to electroforming, and is particularly, although not exclusively, concerned with a method and apparatus for electroforming brass cylinders.
Electroforming is a well known process in which a former or mandrel is electroplated with a metal by using the former as a cathode, to form a metal layer of the desired shape, after which the former is removed. While many metals are electroplated for decorative, protective, and other uses, only a few find use for electroforming, partly because only a few of the processes available are satisfactory for producing thick or heavy coatings of satisfactory physical and mechanical properties. Most electroforming is carried out with nickel or copper, which fulfill most of the engineering requirements. Very little electroforming has been carried out using alloys.
Electroplating, on the other hand, has been carried out using alloys such as brass, but there is no information to suggest that coatings of acceptable quality can be obtained at thicknesses in excess of about 0.0005 inches (12.5 microns). Even if sound thicker coatings of brass were possible, for example 0.005 inches 125 microns) thick, the rates of deposition hitherto achieved are so slow that an inordinate amount of time would be required to achieve substantial thickness. The rate of deposition obtained in a conventional brass plating bath is slow since only relatively small current densities are used. This is because of the low cathode efficiency, that is to say the low efficiency with which metal is deposited. Increasing the current density further in an attempt to increase the deposition rate results in even lower cathode efficiencies. A further, serious, factor to increase current density is that the brass anodes employed in the bath polarize, and eventually become passive. The composition of the bath may then rapidly become out of balance.
The quality of the metal deposit often deteriorates as thickness increases, and one method of improving the quality of the deposit is to employ periodic reversal of the plating current. In addition, in order to ensure uniform deposition around an article during the electroforming thereof, the article may be rotated within the bath. Typically, in such a process, a current density of 3-5 amps/dm.sup.2 has been used. In these circumstances, a time of about 2 hours is necessary to produce a coating of about 0.005 inches (125 microns). This time is rather long for a commercial process.